The invention relates to a solid-state video camera having a brightness control unit which, on the basis of a measured brightness signal of a video picture or image, controls brightness of the image according to a nominal brightness value and has at least one brightness control mode (M1, M2, M3, M4). The invention also relates to a method for brightness control in a solid-state video camera wherein, using a video image recorded by a solid-state image sensor, a brightness signal of the image is obtained and brightness of the image is controlled to a nominal value on the basis of at least one brightness control mode (M1, M2, M3, M4).
Under unfavorable light conditions in medical and technical endoscopy, the applied endoscopic solid-state video camera generally changes from a shutter control mode, in which the exposure time of the pixels is controlled in steps, into a gain control mode, in which gain of the pixels is controlled. In order to permit sufficient image brightness in the gain control mode, gain factors are necessary which unavoidably result in noise on the resulting image. This noise has a negative effect on the image resolution and the recognition of detail. Furthermore it has a disturbing effect on the user.
In order to counteract this, some endoscopic solid-state video cameras have a so-called integration mode. In the integration mode, in a video frame sequence (2 frames at 1/50 second) the image sensor is not read twice, but only once, which corresponds to a doubling of the exposure time. When required, the exposure time may continue to be increased to a multiple of this frame exposure time. This procedure increases the brightness according to the number of integrated images without amplifying the noise component. The image repetition rate is, however, reduced accordingly. The reduction of the image repetition rate has the effect of slow-motion type jumps in the video. Additionally, moving objects appear significantly out of focus which is known to occur in photography when using slow shutter speeds.
Through the image integration the temporal and spatial resolution is thus lost with moving endoscopic sequences. In extreme cases meaningful endoscopy is no longer possible.
One example of a conventional solid-state video camera common in medical endoscopy, and the function of its brightness control, are explained with regard to the accompanying FIGS. 4 and 5. The conventional solid-state video camera shown in FIG. 4 as a block diagram, and indicated generally with reference numeral 20, comprises an image sensor 22, a CDS/gain unit 23, an A/D transducer 24, a digital signal processor 25, a time control unit 26, a micro-controller 28, a D/A transducer unit 31, a decoder 32 and output circuits 33.
The digital signal processor 25 of the known solid-state video camera 20 shown in FIG. 4 carries out digital video signal processing. The micro-controller 28 serves for camera control and, in combination with the signal processor, serves for brightness control. Additionally, in FIG. 4 there is shown an optional memory unit 27. This is necessary in case that a prolonged exposure is realized. The micro-controller 28 is in active connection with the digital signal processor 25, the memory unit 27, the time control unit 26 and the CDC/gain unit 23.
The brightness control of the video image of a solid-state video camera uses a reading gained from an adjustable measurement window in the video image by way of forming averages, peak value recognition, histogram evaluation or a combination of these methods for regulating the image brightness by way of continuous comparison to an adjustable nominal value.
The diagram shown in FIG. 5 schematically and graphically shows the course of the brightness control of a conventional endoscopic camera. On the left vertical axis of the diagram exemplary gain factors are plotted, and on the right vertical axis the shutter times are plotted. The horizontal axis indicates the (measured) object brightness in relative units.
The step function shown by way of an unbroken line is the control function in the shutter mode M2. The stepwise shutter control is, for example, effected in 6 dB steps. The step function of the shutter control M2 goes from a greater object brightness with a short shutter speed in the left part of the diagram up to the longest shutter speed 1/50 s (at NTSC standard 1/60 s). A further extension of the shutter speed is not possible with conventional video cameras.
So that the dynamics may be increased further, the gain of the camera sets in from a threshold value S2 of the object brightness. This so-called gain control mode M3 is drawn in by way of a dotted line in FIG. 5. As an option, as already drawn in FIG. 5, image integration may be activated earlier at a higher threshold value S3. This brightness control mode M4 is illustrated by a dot-dashed line in FIG. 5. The image integration is effected in 6 dB steps. In the diagram shown in FIG. 5 the image integration is limited by the control mode M4, for example, to four images. The gain function according to the brightness control mode M then begins to set in.
Not shown in FIG. 5 is a combination of shutter control (mode M2) and gain control (mode M3) wherein the jumps in brightness on account of the shutter control M2 are compensated by a gain control M3. It is likewise known to let the individual operating modes blend into one another in a flowing manner at the transition points.